This invention relates to a carbon/metal composite. More particularly, it relates to a carbon/metal composite having a high strength, good wear resistance, and good electrical properties and which is suitable for use as a sliding current collector such as a pantograph slider for electric trains.
In the past, carbon materials and metallic materials such as cast alloys and sintered alloys have been employed for sliding current collectors. Metallic materials have the advantages of high strength and good electrical conductivity, but they have the drawbacks that they produce many arcs, their sliding properties are inadequate, and they produce a great deal of wear in the trolley wires or third rails with which they are in sliding contact. Carbon materials have excellent lubricating properties and produce little wear of the wires or rails which they contact, but they have a high electric resistance and easily heat up during current collection.
The power consumption of railroad cars has been increasing due to higher train speeds and the installation of air conditioning. In order to cope with the increased power consumption, in recent years, sliding current collectors have begun to be made from carbon/metal composites, which combine the excellent sliding properties of carbon with the electrical conductivity of metals.
Japanese Published Examined Patent Application No. 56-14732 (1981) discloses a carbon/metal composite which is formed by impregnation under pressure of a metal into the pores of a carbon material.
Japanese Published Unexamined Patent Application No. 60-238402 (1985) discloses a current collecting material made from a carbon material containing metal powder which is obtained by adding a metal powder with good conductivity to a carbon raw material.
Japanese Published Unexamined Patent Application No. 61-245957 (1986) discloses a manufacturing method for a current collecting material in which a mixture of a carbon aggregate with a binder which contains metal fibers and/or carbon fibers is molded and baked, after which the baked body is impregnated with a metal.
Japanese Published Unexamined Patent Application No. 62-72564 (1987) discloses a manufacturing method for a sliding current collecting material in which metal fibers are blended into a carbon raw material, and the blend is molded at ambient temperature and then baked.
Japanese Published Unexamined Patent Application No. 62-197352 (1987) discloses a manufacturing method for a sliding current collecting carbon material in which metal fibers are blended with a carbon raw material so as to be oriented unidirectionally, after which molding and baking are performed.
Japanese Published Unexamined Patent Application No. 63-215731 (1988) discloses a manufacturing method for a carbon/metal composite frictional material for use in brakes in which pitch, metal fibers, and graphite are mixed and then molded at a temperature of 450.degree.-600.degree. C. under a molding pressure of at least 40 kg/cm.sup.2.
However, the great majority of carbon/metal composites produce a great deal of wear by sliding contact with trolley wires, normally made of copper, when the surfaces of the wire are in a toughened condition. Such a roughened surface of trolley wires will be usually observed while pantograph sliders formed of a sintered metal which are at present prevailing is being replaced by those of a carbon/metal composite in the future, i.e., during a period when sliders of these two types are used concurrently.
Furthermore, most of the conventional carbon/metal composites are inferior to conventional metal sliders with respect to bending strength and impact strength. It is easy for sliders made from carbon/metal composites to be chipped or broken by collision with a hanger ear of a trolley wire which has been detached due to vibration or shock caused by running trains or any other accidental cause, so they are less safe and less reliable than conventional metal sliders.
Furthermore, carbon/metal composites have a higher electric resistance than conventional metal sliders. A high electric resistance leads to an increase in the temperature of trolley wires due to Joule heating. The temperature increase is particularly significant when a train is stopped but the air conditioning and interior lighting of the train are still operating. A high temperature may cause the breakage of the trolley wires under high tension, which is extremely dangerous.
It is possible to improve the strength and electrical properties of a carbon/metal composite by increasing the metal content of the composite. However, as the metal content is increased, more sparks are generated between the slider and the trolley wires, and the wear of the trolley wires and the slider is increased, which is undersirable.